1. Field of the Invention
The present invention relates to an optical beam scanning device which causes a luminous flux from a light source to scan a photoconductive surface of a photoconductor in the main scanning direction, and particularly a technique to realize improvement in optical characteristics.
2. Description of the Related Art
Traditionally, for an optical beam scanning device which shapes a luminous flux from a light source into a predetermined sectional shape and causes the luminous flux to scan in a predetermined direction, there is known a technique of providing a lens or diffractive optics with negative power that passes luminous fluxes from plural light sources in a pre-deflection optical system, thereby changing the beam spacing in a direction of restraining color shift which occurs in an image forming apparatus having the optical beam scanning device when a temperature change occurs (JP-A-2005-221870).
Also, there is known a technique of providing a diffractive optics which corrects chromatic aberration of magnification in a pre-deflection optical system, thus preventing the length of a scanning line in the main scanning direction from being varied by a change in wavelength (JP-A-2006-171117).
Moreover, a configuration is known in which a scanning optical device having a refraction surface and a diffraction surface is provided in a post-deflection optical system, the radius of curvature in the sub scanning direction on at least one refraction surface of the scanning optical device is continuously changed from the optical axis toward outside in accordance with the main scanning direction, and the diffraction power in the sub scanning direction of at least one diffraction surface is continuously changed from the optical axis toward outside in accordance with the main scanning direction (JP-A-2002-221681).
However, in the technique descried in the JP-A-2005-221870, the number of lenses arranged in the pre-deflection optical system must be increased.
Also, in the techniques described in JP-A-2006-171117 and JP-A-2002-221681, since correction of color shift due to thermal expansion of the image forming apparatus is considered with respect to the sub scanning direction, there is a problem that color shift occurs due to a temperature rise in the image forming apparatus.